A phenolic resin has been widely used for mixing in the kneading of carbon-containing bricks. A phenolic resin used as a binder in kneading a mixture containing carbon, such as graphite, exhibits excellent properties in kneading and molding and a high residual carbon on calcination.
However, use of a phenolic resin involves the following drawbacks or problems.
Firstly, a phenolic resin gives off decomposition gases, such as water, hydrogen, ethylene, phenol, cresol, and xylenol, when carbonized in a temperature range of from 350.degree. C. to 650.degree. C. Of these components, in particular, phenol, cresol, and xylenol cause odor. For example, carbon-containing unburned bricks for use as a furnace lining material are generally supplied as a product after only a drying treatment at about 200.degree. C., laid to form the inner wall of a furnace, and preheated to about 1000.degree. C. before actual use. In the preheating treatment, the phenolic resin decomposes to generate decomposition gas, which is discharged out of the furnace to cause air pollution and odor, etc.
Secondly, where a phenolic resin is used as a binder, the resulting structure is dense and has insufficient open cells because of the excellent molding properties of the phenolic resin. Therefore, the structure is liable to destruction due to evolution of decomposition gas on heating. That is, the brick after hardening by drying has a structure with fewer open cells and little air permeability. Such a structure, while contributing to improvement in brick strength after drying, hinders escape of gas generated by decomposition of the phenolic resin at temperatures higher than the drying and hardening temperature. It follows that the internal pressure increases, leading to destruction of the structure or cracking. This phenomenon becomes more conspicuous as the rate of temperature rise increases or the size of the brick increases. In order to prevent the phenomenon it is necessary to strictly control the rate of temperature rise until the temperature reaches a range in which the phenolic resin no more generates decomposition gas. In the case of unburned bricks, however, since the heat treating temperature for obtaining a product is generally from 200.degree. C. to 300.degree. C., it is difficult to have decomposition gas completely removed before the bricks are supplied as a product. In the production of carbon-containing burned bricks, too, strict temperature control in the drying and burning steps involves many technical and economical problems.
Thirdly, carbon produced from a phenolic resin is glassy carbon inferior in resistance to spalling. That is, where carbon-containing bricks are produced by using a phenolic resin as a binder, although the phenolic resin has a high residual carbon on burning, the resulting products have poor spalling resistance.
Therefore, use of a phenolic resin is not preferred in the production of carbon-containing bricks which are required to have spalling resistance, and carbon-containing bricks which do not use a phenolic resin have been sought. Pitch-bonded type bricks using pitch as a binder are known to meet the demand. However, pitch-bonded carbon-containing bricks, while excellent in spalling resistance, are disadvantageous in that pitch, which is solid at ordinary temperatures, must be heated above the softening point and kneaded (i.e., heat kneading). Besides, pitch is inferior to a phenolic resin in kneading properties and molding properties.
A process comprising uniformly mixing a refractory matrix material and fine powder of a carbon element, such as carbon black, together with an aqueous solution or suspension of a non-aromatic organic high-molecular compound, such as polyacrylate, vinyl polymers, alcohol polymers, and methyl cellulose, has been proposed as a process for producing carbon-containing bricks without using a phenolic resin nor pitch (see a Japanese national phase published patent application No. Hei 7-504641). According to this technique, however, the organic high-molecular compound such as polyacrylate used in the form of an aqueous solution or suspension has poorer kneading properties than a phenolic resin, and the resulting molded articles have a low bulk specific gravity. Further, the binder is inferior to a phenolic resin in kneading properties and molding properties.
It is also known that hexahydric alcohols, such as sorbitol, mannitol, and polysorbitol, are useful as a binder of carbon-containing bricks (see JP-A-52-32912, the term "JP-A" as used herein means an "unexamined published Japanese patent application"). However, when these binders are used, it is difficult to obtain products equal in anticorrosion to those produced by using a phenolic resin.
As stated above, the prevailing phenolic resin is excellent in kneading properties and molding properties notwithstanding the above-mentioned first to third drawbacks. There is no binder that is superior to a phenolic resin in kneading properties and molding properties and yet economical. Accordingly, it is the state of the art to employ a phenolic resin, accepting the first to third drawbacks involved in it.
The present invention has been reached in the light of the disadvantages of the pitch-bonded type, the use of a non-aromatic organic high-molecular compound, and the use of a hexahydric alcohol as well as the first to third drawbacks associating the use of a phenolic resin.
An object of the present invention is to provide carbon-containing refractories which give off little decomposition gas having an offensive odor, exhibit excellent spalling resistance, and are equal or superior in kneading properties and molding properties to the refractory materials containing a phenolic resin as a binder; and a process for producing the same.